Well a bunch of answers. Less tension on strings and neck. Could be for various reasons from concerns about guitar strength to perference of feel.You could put some fat strings on with no extra stress on guitar. Yould might use open tunings that would be too much if you tuned individual strings up in standard tuning.You might find vocals like to be a half step below open key that you like chords of. You might like that loose feel. If it's not a concern about guitar strength, play with it. Just know that unless you use fatter strings you are going to lose volume and probably will need guitar set up for that.The lesser amount of tension won't shake the top as much and also will let the neck drop back a little which may cause buzzing this neck would need adjusted. Simple. The most common time I see guitars tuned down is with 12 strings where the stress of all those string is a lot.